This invention relates to an optical filter for attenuating incident laser radiation and, more particularly, to a filter for absorbing neodymium YAG-doubled laser radiation at 532 nanometers (nm).
In certain applications, it is necessary to attenuate incident laser radiation at one or more laser wavelengths, while at the same time transmitting a substantial portion of the incoming radiation at other wavelengths. One such application involves visors worn by military personnel. Because of the extremely high intensity of the laser radiation, the attenuation at laser wavelengths must be correspondingly high: optical densities of 3 or more at the laser wavelength are typical for filters of this type. In order to satisfy these twin requirements of high attenuation at the laser wavelength and substantial transmission at adjacent wavelengths, the filter must have an extremely sharp cutoff characteristic.
Optical filters are commonly made by incorporating one or more compounds, selected for their absorption characteristics, into a suitable light-transmissive host material--in particular, a polymeric matrix such as polycarbonate. To be useful for such an application, the absorber must have several properties besides absorption at the desired wavelength. The compound should be soluble in the host material, and should be compatible with the host material and any other additives. The compound should be sufficiently stable to permit its incorporation and use in the desired host material without excessive degradation. Finally, the compound should be capable of being readily synthesized on the scale desired.
McKoy et al U.S. Pat. No. 4,622,174 discloses a transparent protective laser shield containing metalloporphyrins--more particularly, platinum octaethylporphyrin (PtOEP) for absorption of neodymium YAG-doubled laser radiation at 532 nm and vanadyl phthalocyanine (VOPc) for absorption of ruby laser radiation at 694 nm. Gordon U.S. Pat. No. 4,657,345 discloses a similar shield in which the absorbers are diffused into a surface of the host material rather than being disseminated uniformly through it. Although PtOEP absorbs strongly at 532 nm, its absorption maximum does not coincide with that wavelength, but occurs at a slightly longer wavelength, at about 537 nm. Because of the extreme sharpness of the absorption peak, this implies that more of the absorber must be used to achieve a given optical density at the laser wavelength than if the absorption maximum coincided with the laser wavelength. Not only is the absorber relatively expensive, because of the platinum used, but the resulting filter will have a lower transmittance at other wavelengths because of the greater amount of material used.